Available-for-Sale (AFS) Securities (Interest Rate Payment Terms) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,297,647		$ 4,518,370
Total Non-MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,410,900	[1]	901,993
MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,886,747		3,616,377
Fixed Interest Rate [Member] | Total Non-MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	660,962		751,993
Fixed Interest Rate [Member] | MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,153,469		2,145,595
Adjustable Interest Rate [Member] | Total Non-MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	749,938		150,000
Adjustable Interest Rate [Member] | MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,733,278		$ 1,470,782

[1] Amortized cost includes adjustments made to the cost basis of an investment for accretion and/or amortization, collection of cash, and/or previous OTTI recognized in earnings.